The advantages of employing a combination anchoring line consisting of the lower length of chain cable serially connected to an upper lenth of wire rope are well recognized. In particular, better anchoring characteristics at certain water depths can be achieved than possible through use of wire rope or chain alone, and an overall capability of mooring in deeper waters obtained. In handling such an anchor line directly from a floating structure, a winch and a windlass are required to handle the components of the anchoring system, and in "disconnect" systems in which the junction between the wire rope and chain cable is broken, to permit the appropriate component of the anchor line to be handled by the appropriate one of the winch and windlass, provision must be made for transferring chain load between the winch and windlass.
In U.S. Pat. No. 3,842,776 which issued to Wudtke on Oct. 22, 1974, there is described a particulate disconnect system in which a chain hanger consisting of a length of wire rope fastened to the anchored structure and teminated with a hook is taught as means for supporting chain load when the connection between wire rope and chain cable is to be broken, during paying out or retrieval of the anchor line. For example, during paying out, the chain is delivered from a chain locker aboard the anchored structure by means of a chain wheel having a lead chain to which the chain cable is connected. Once the anchor line is payed out to the extent possible by the windlass, the chain hanger is attached to the chain to support its weight, the windlass lead chain, disconnected from the chain cable by means of a disconnectable link, and the chain cable, then connected to a lead chain attached to the wire rope line. The chain hanger is then disengaged, and the anchor line payed out under the control of the winch. Such disconnect systems are satisfactory, and are still used today, but suffer the serious disadvantage that chain hangers are cumbersome and endanger the safety of the workmen who must operate them, particularly in rough waters.
An alternative disconnect system which proposed the elimination of a chain hanger is disclosed in U.S. Pat. No. 4,020,779 which issued on May 3, 1977 to Kitt. The Kitt patent describes an anchoring system substantially identical to that of the Wudtke patent, except that a special chain link with three loops (commonly referred to as a "tri-link") is introduced into the chain line. Two loops are used at any time to couple the chain cable to the winch or windlass. Both the wire line and windlass have an associated length of lead chain terminated with a disconnectable link by means of which the particular lead chain, if not already under load, can be coupled to the extra loop of the tri-link. Thus, both lead chains could be simultaneously engaged in the tri-link because of the special additional loop, and loads transferred directly between winch and windlass without intermediate transfer of anchor chain load to a chain hanger.
As a load transfer device, the tri-link closely resembles a "tri-plate" commonly used in earlier Union Purchase rigs, and in early semisubmersible drill rigs in which a combination anchor line was not run directly over a fairlead sheave. Such load transfer devices were never designed to run over sheaves and wildcats. The tri-link, somewhat resembling a more conventional chain link, can accordingly pass smoothly over a sheave or chain wheel depending in large measure on how the tri-link is oriented during its approach to sheave or chain wheel. As taught in the Kitt patent, a swivel is required at least in the lead chain associated with the wire lead, and the chain links extending from the chain locker must be aligned in a specific manner, to ensure that the extra loop of the tri-link passes over the associated fairlead sheave and chain wheel with the extra loop pointing radially out. A significant problem with such an arrangement is that a swivel cannot be properly conveyed over a sheave. A swivel may be expected to be severely stressed, and there is a serious risk of failure of the chain line. Also it cannot be guaranteed that the chain cable will be conveyed over the chain wheel consistently in a predetermined orientation, as chain links are known to advance a half or full pitch during passage over a chain wheel, particularly during deployment, causing all succeeding links to be conveyed rotated at 90 or 180 degrees to the attitude otherwise expected. Additionally, the trilink is unsuitable for systems incorporating a pocketted fairleader intended to permit the associated vessel to moor for extended periods of time on chain alone, which is particularly desirable at certain water depths.
It is an object of the present invention to provide a combination anchor system of the disconnect type incorporating into the chain line a load transfer device which eliminates need for a chain hanger and which in various embodiments can be conveniently and reliably conveyed over a smooth fairlead sheave, a pocketted fairlead sheave or a chain wheel.